With such metal cutting tools, the cutting edge of the tip is defined between a rake surface thereof and a relief flank or clearance face and, in use, the cutting edge and the portion of the relief flank adjacent thereto, become rapidly worn and, where the tip is formed on a replaceable insert, the latter must therefore be frequently replaced or rotated so as to index a fresh cutting edge in position. Similarly, where the tip forms an integral part of the cutting tool, the tip must be frequently reset or the entire tool must be replaced.
With such cutting tips there is generally specified (by the manufacturers) a maximum degree of wear of the relief flank after which replacement, resetting or indexing must be effected. This maximum degree of wear is usually specified in terms of the maximum width VB.sub.max of wear of the relief flank when measured from the cutting edge. Thus, when the flank wear reaches the specified maximum VB.sub.max, the tip should be reset or replaced.
It is well-known that one of the factors determining flank wear is the hardness of the cutting tip material. Thus, the harder the material the less will be the rate of wear and the longer the cutting tip can be used before wear has reached VB.sub.max requiring replacement. However, this hardness is directly related to the working temperature and with rise of working temperature there is an associated reduction in hardness, with a consequent increase in the rate of wear.
It is for this reason that, in the use of such cutting tools, means have been provided for the fluid cooling of the cutting tip in the region of the cutting edge thereof.
In one known method of fluid cooling, the upper surface of the cutting tip or insert is sprayed with a coolant fluid but it is found that the continued movement of the chip over the rake surface of the cutting insert disturbs the effective cooling of the insert by the coolant.
In another known method of cooling, a coolant is passed through the cutting tip from the base thereof to the upper rake surface. It is found in practice, however, that the pressure of the chip on the upper rake surface prevents the coolant from effectively reaching the region of the cutting edge, and here again the efficiency of cooling is limited.
A further method which has been employed has been to direct a coolant through a duct formed in the cutting insert or cutting tip, which duct emerges from an aperture in the relief flank of the cutting insert. It is found, however, that in order to ensure effective distribution of the coolant liquid around the relief flank and in the vicinity of the cutting edge, a number of such coolant ducts must be provided and this, in turn, leads to a significant weakening of the cutting tip. Furthermore, the penetration of the coolant is hindered by the narrow gap between the relief flank and the workpiece.
It has also been proposed to direct a jet of a coolant liquid into the space between the relief flank and the workpiece, but this method is of very limited value, particularly when the tool involved is a grooving or parting tool, seeing that the region between the relief flank and the workpiece is of extremely limited accessibility as compared with a turning tool.